President Barack Obama shed tears Tuesday as he announced limited measures to tackle rampant US gun violence and called on Americans to punish lawmakers who oppose more meaningful reforms.

Flanked by survivors of the gun violence that kills around 30,000 Americans every year, and relatives of those killed, Obama became emotional as he remembered 20 elementary school children shot dead three years ago in Newtown, Connecticut.

"Every time I think about those kids, it gets me mad," the president said, struggling to collect himself. "So all of us need to demand a Congress brave enough to stand up to the gun lobby's lies."

Obama was unveiling a handful of executive measures that will make it harder to buy and sell weapons, but which he admitted would not stop the scourge of mass shootings.

"We know we can't stop every act of violence, every act of evil in the world. But maybe we could try to stop one act of evil, one act of violence," he said.

There are thought to be substantially more than 300 million guns in circulation in the United States, more than one per person.

'A topic that gets Obama more emotional than any other'

Speaking in the East Room of the White House, Obama invoked the words of his hero, Martin Luther King, as he called for urgency in a generational struggle akin to the fight for women's, African-American, or gay rights.

"We do have to feel a sense of urgency about it. In Dr. King's words, we need to feel the fierce urgency of now, because people are dying," he said. "And the constant excuses for inaction no longer do."

"Yes, it will be hard. And it won't happen overnight. It won't happen during this Congress. It won't happen during my presidency," he said.

"But a lot of things don't happen overnight. A woman's right to vote didn't happen overnight. The liberation of African-Americans didn't happen overnight. LGBT rights, that was decades worth of work."

Gun lobby 'can't hold America hostage'

Taking on Republicans directly, he decried the gun lobby's grip on Washington.

"The gun lobby may be holding Congress hostage right now, but they can't hold America hostage," he said.

Polls have shown most Americans back tougher gun laws.

But that support has ebbed recently amid concerns about the Islamic State group and the wider threat from terrorism.

Republicans, who have long championed gun owners, quickly castigated Obama for trampling on the constitutional right to bear arms.

"His words and actions amount to a form of intimidation that undermines liberty," said the speaker of the House of Representatives, Paul Ryan.

"No matter what President Obama says, his word does not trump the Second Amendment. We will conduct vigilant oversight. His executive order will no doubt be challenged in the courts."

The measures would tighten rules on who must register as a gun dealer, narrow a "gun show" loophole that allows buyers to dodge background checks, and crack down on "straw purchases" of weapons through intermediaries.

It would also encourage the Pentagon, with its vast buying power, to procure weapons from manufacturers who invest in "gun safety technology," such as fingerprint scanners that could be applied to commercial weapons.

'Gun control now likely to feature prominently in 2016 campaign'

Obama's remarks and recourse to controversial executive actions during an election year ensure that gun control will feature prominently in the 2016 race for the White House.

In the past, both Democrats and Republicans have used the issue to invigorate supporters and raise campaign funds.

Gun manufacturers frequently see sales surge each time tighter gun controls are proposed or a mass shooting occurs.

According to FBI figures, requests for background checks jumped by a third to over three million in December.

Gunmaker Smith and Wesson on Monday hiked its earnings estimates for the year ending in April, citing data which point to increased sales.